This application relates generally to data storage. More specifically, this application relates to removable-cartridge data-storage devices.
Computer backup has traditionally been performed using tape-drive technologies. This has been true for a number of different reasons, one of which is the fact that tape media have traditionally provided the lowest cost per bit of storage. In addition, tape devices had the advantage that they used removable tape cartridges, which may be taken to an off-site location for disaster recovery. This traditional emphasis on tape-based storage technologies has resulted in a significant installed base of hardware and system software for accommodating these technologies. For instance, a current trend uses mechanical automation systems, such as tape-cartridge autoloaders and tape libraries holding multiple removable tape cartridges, with multiple tape drives being embedded inside a storage area.
More recently, increased attention has been focused on magnetic hard disk drives (“HDDs”) as a possible alternative to tape-based systems. This attention has been prompted by a steady decline in the cost of disk-based storage concomitant with increases in its storage capacity and performance, making it increasingly competitive with tape. But accommodating the existing installed configurations for handling tape-based systems remains a challenge given the different storage techniques used by the different media. For instance, because tape drives are sequential-access devices, new disk-based systems designed for backup applications that cannot emulate tape drives by implementing a sequential-access command protocol are likely to fail.
Both tape drives and disk systems also implement some type of error correction coding (“ECC”) to reduce the risk of data loss. Current tape drives implementing sophisticated multilevel ECC algorithms specify uncorrectable error rates on the order of 1 error in 1017 bits read. Disk drives currently have more limited ECC designs and typically specify uncorrectable error rates on the order of 1 error in 1014 bits read. A common feature of these systems, however, is the inflexible way that error correction is handled. Formats are typically used that provide error-correction capabilities based on average or worst-case scenarios, fixing the error-correction capability in the format. Such formats are thus unable to easily accommodate differences in environmental factors, or even differences in error-rate requirements for different applications. This uniformity thus sometimes results in undesirably limited error-correction capability for some applications while at the same time resulting in excessive (and therefore inefficient) error-correction capability for other applications.
There is accordingly a general need in the art for methods and systems that provide error correction without such deficiencies. This need is especially acute in disk systems that apply tape emulation.